Any rigid pipe that is suitable for water and doesn’t have any fittings in it will work. L (Blue) is what you want to use if you have copper and is to code but PVC will also work and is becoming more common - the key is that the pipe is straight and have nothing in it that could create an eddy or chaotic current because these will give you inaccurate / inconsistent flow readings. The purpose 6"-12" of straight pipe is just to acquiesce the flow.
